In the meat processing industry, freshly slaughtered animal carcasses are typically chilled before cutting and further processing. Chilling reduces bacterial growth and retards the onset of spoilage, thereby increasing shelf life of the meat product. A uniform carcass temperature of about 34.degree.-38.degree. F. is generally desired so that the meat and fat are firm enough for smooth cutting. The difficulty of chilling carcasses from a fresh-killed temperature of about 100.degree. F. to a temperature of about 35.degree. F. on a continuous basis with efficiency has been a long-standing problem in the industry.
Various difficulties have been associated with the methods and equipment of the prior art. For example, conventional refrigeration equipment capable of generating low temperatures at high rates is relatively complex and thus expensive and difficult to maintain. Such equipment operates by circulating large volumes of chilled air around carcasses placed in a cooler for extended periods of time on the order of 12 to 16 hours. Since this process is relatively slow, cooler capacity often has been the limiting factor in the output of a meat processing plant. Further, the possibility of bacterial contamination is greatly increased under such conditions, and substantial tissue shrinkage occurs due to surface dehydration of the meat. Although a relatively high humidity is maintained in an attempt to reduce dehydration of the meat, much of the chilling action in such equipment results from evaporation of moisture from the meat's surface. This is a significant problem adversely affecting profitability because about 1%-3% or more of the fresh-killed carcass weight is typically lost to dehydration. In addition to being slow and causing carcass shrink, conventional refrigeration equipment consumes much floor space and is inefficient and costly to operate.
More recently, there have been several attempts to utilize cryogenic fluids in the refrigeration of meat. Cryogenic fluids such as liquid carbon dioxide, air or nitrogen range in temperature between -100.degree. F. and -400.degree. F. and are suitable for use as refrigerants by reason of their high cooling capacities. However, prior attempts to utilize such fluids have been confined primarily to injection of cryogenic liquids into a chamber to effect refrigeration by displacing the gas therein with vaporized cryogenic fluid. For instance, U.S. Pat. No. 3,492,831 to Maurer et al. discloses a meat refrigeration and dehumidification system wherein liquid nitrogen is dispensed and vaporized inside a closed chamber to cool the gas circulated in contact with the meat therein. In U.S. Pat. No. 3,769,807 to Foster, liquid carbon dioxide is injected into an insulated chamber to maintain the atmosphere therein at a desired temperature.
Other approaches have involved direct contact between the meat and cryogenic fluid. Small poultry carcasses are conveyed through a slurry of carbon dioxide particles and gas in U.S. Pat. No. 3,468,135 to Doll et al. Individual cuts of meat are quick frozen by immersion in a cryogenic liquid bath in the machine shown in U.S. Pat. No. 3,832,864 to Rasovich. In addition, there have been attempts to effect chilling by spraying cryogenic fluid directly on substantially whole carcasses. Heretofore, however, there has not been a chilling process involving the step of immersing entire animal carcasses in a bath of cryogenic liquid to accomplish partial chilling, followed by the step of holding the carcasses in chilled gas circulating at a high rate to complete chilling.
Thus, a need has arisen for a more efficient technique for chilling substantially whole carcasses to a desired cooler but unfrozen temperature by means of immersion in an expendable cryogenic fluid.